1. Field of the Invention
The present invention relates to methods and apparatus for the recovery of hydrocarbons. In another aspect, the present invention relates methods and apparatus for the recovery or the upgrading of hydrocarbons utilizing in situ combustion.
2. Description of the Related Art
In many parts of the world reservoirs are abundant in heavy oil and tar sands. For example, those in Alberta, Canada; Utah and California in the United States; the Orinoco Belt of Venezuela; and the U.S.S.R. Such tar sand deposits contain energy potential estimated to be quite great, with the total world reserve of tar sand deposits estimated to be 2,100 billion barrels of oil, of which about 980 billion are located in Alberta, Canada, and of which about 18 billion barrels of oil are present in shallow deposits in the United States.
Conventional recovery of hydrocarbons from heavy oil deposits is generally accomplished by steam injection to swell and lower the viscosity of the crude to the point where it can be pushed toward the production wells. In those reservoirs where steam injectivity is high enough, this is a very efficient means of heating and producing the formation. Unfortunately, a large number of reservoirs contain tar of sufficiently high viscosity and saturation that initial steam injectivity is severely limited, so that very little steam can be injected into the deposit without exceeding the formation fracturing pressure. Most of these tar sand deposits have previously not been capable of economic production.
In steam flooding deposits with low initial injectivity the major hurdle to production is the confinement of steam along preferential flow channels between injection and production wells. Several proposals have been made to provide horizontal wells or conduits within a tar sand deposit to deliver hot fluids such as steam into the deposit, thereby heating and reducing the viscosity of the bitumen in tar sands adjacent to the horizontal well or conduit. U.S. Pat. No. 3,986,557 discloses use of such a conduit with a perforated section to allow entry of steam into, and drainage of mobilized tar out of, the tar sand deposit. U.S. Pat. Nos. 3,994,340 and 4,037,658 disclose use of such conduits or wells simply to heat an adjacent portion of deposit, thereby allowing injection of steam into the mobilized portions of the tar sand deposit.
U.S. Pat. No. 4,344,485 discloses a method for continuously producing viscous hydrocarbons by gravity drainage while injecting heated fluids. One embodiment discloses two wells which are drilled into the deposit, with an injector located directly above the producer. Steam is injected via the injection well to heat the formation. A very large steam saturation volume known as a steam chamber is formed in the formation adjacent to the injector. As the steam condenses and gives up its heat to the formation, the viscous hydrocarbons are mobilized and drain by gravity toward the production well (steam assisted gravity drainage or "SAGD"). Unfortunately the SAGD process is limited because the wells must generally be placed fairly close together and is very sensitive to and hindered by the existence of shale layers in the vicinity of the wells. Also, the formation of water-in-oil emulsions which are more viscous than the original bitumen and may slow productivity with steaming methods.
As disclosed by Chu in SPE Paper No. 9772 and SPE Paper No. 9994, the in situ combustion process, ever since its inception in the mid-thirties, has proven to be a significant method for recovering oil, especially heavy oil, and may be undertaken for primary, secondary and tertiary recovery of crude oil, and is employed in situations where the reservoir characteristics and crude oil properties economically justify this recovery approach.
In a conventional in situ combustion process, an oxidant is injected into an input well and combustion is either self-initiated or is initiated by one of many well known methods. It is ideally hoped that the zone of combustion will move as a radial front from the input well and drive the reservoir oil ahead of it to the production well.
U.S. Pat. No. 4,597,441 to Ware et al., discloses a prior art variation on the conventional in situ combustion recovery process, an in situ hydrogenation process in which the hydrogenation temperature is achieved by means of in situ combustion.
In addition to helping produce hydrocarbons, the in situ combustion process has also been used to upgrade or crack hydrocarbons.
Some crude oils are of such low quality and high viscosity that they are produced only with difficulty at a substantially increased expense over light crudes. And once they are brought to the surface they must be prerefined to reduce asphaltic constituents and inorganic catalyst poisons at a cost amounting to as much as fifty percent of the well head price of the oil in order to put them in condition for conventional refining. It would be economically desirable if such an oil could be pretreated in the reservoir and produced as a prerefined upgraded oil.
Upgrading is a relative term which is used to indicate an increase in both quality and value. The upgraded oil recovered from the reservoir will contain a greater proportion of the more valuable lower boiling distillate material and a smaller amount of the less desired high boiling and asphaltic fractions than the virgin oil and may contain only distillate products.
U.S. Pat. No. 3,332,489 to Morse discloses a process for upgrading oil by in situ combustion, which generally comprises injecting oxidizing gas at a high rate into only the bottom of an oil bearing formation, burning out in situ the upper portion of the formation, reducing the rate of the gas injection to stabilize the combustion front and vaporize an upgraded oil product, transporting the vaporized product through the burned out upper portion of the formation, through perforations adjacent only to the top of the formation and into a remote output well and producing to the surface the fluids entering the output well.
While current methods exist for the recovering and upgrading hydrocarbons which utilize in situ combustion, the current methods suffer from several defects. Most notably, the present in situ combustion methods tend to generate in situ combustion gases faster than they can be vented from the reservoir, thus limiting the rate of combustion propagation. The success of any in situ combustion scheme relies heavily on the ability to consistently and simultaneously produce hydrocarbons and vent in situ combustion gases from the formation. Also, with in situ upgrading, the hydrocarbons surrounding the high temperature region of the combustion front becomes high mobile and generally tends to flow toward the producer before it can be reached by the approaching combustion front and upgraded. As a result, only a very small fraction of the produced oil is submitted to the high temperatures necessary to crack and upgrade the oil.